OTTAWA — The Conservative Party launched a "groundless, strategic assault" against the labour movement, the head of Canada’s largest private sector union said Sunday after the Tories accused the NDP of accepting improper sponsorships from trade unions.

Wayne Hanley, the national president of UFCW Canada, issued a press release Sunday evening after The Huffington Post Canada first reported that the Conservative Party had asked Elections Canada to investigate whether the NDP had broken the law by accepting money from unions to advertise at its national convention in Vancouver last June.

The allegations, Hanley said, are "a groundless, strategic assault on the labour movement, and on ordinary Canadians who are fed up with the vicious, partisan politics the Harper Conservatives are so proud of."

Hanley said the Tories are well aware that ads at fair market value are allowed and there was no election law breach.

"There is nothing amiss here,” he said. “UFCW Canada respected every rule and regulation in an open and completely transparent manner. It is cynical timing, so typical of the Conservatives, to launch this smear on working Canadians and their unions on the eve of the annual celebration of the achievements of the labour movement."

Hanley suggested the Tories were reacting to news that support for the NDP was rising following the public outpouring of sympathy after leader Jack Layton’s death from cancer on Aug. 22.

The Conservative Party released a letter to the media Sunday morning that its lawyer Arthur Hamilton had written to Marc Mayrand, the chief electoral officer of Canada on Aug. 31, 2011, requesting a review of what it suggested were inappropriate NDP sponsorships.

Hamilton said his client was concerned that prominently displayed signage at the NDP convention suggests sponsorship had been provided by unions, such as USW, UFCW, PSAC and CUPE among others, as well as by Thistle Printing, an incorporated identity.

Posters also suggested the United Steelworkers had sponsored a $300 per-person reception with the NDP leader, Hamilton added.

According to an excerpt from a 2006 letter the Conservative Party received and shared, the Commissioner of Elections Canada wrote that the provision of money, property or service for a convention, minus the fair market value of any tangible benefit one receives in return, constituted a contribution.

The Tories believe that broad definition of contribution means the NDP and some labour groups bent the rules.

“It appears that the NDP has received what the Commissioner of Elections Canada has deemed to be contributions in contravention of the Elections Act,” Hamilton wrote. “The Conservative Part of Canada would therefore request that your office take the steps it deems necessary to review this matter fully and identify any contributions which must be returned.”

In an email, the NDP's fundraising and membership director Heather Wilson said the Conservatives' allegation was unfounded.

"This is a bit rich coming from the In-and-Out party. All rules and regulations have been followed by the NDP. And unlike the Conservatives, we stay within both the spirit and letter of the law. Advertisments at fair-market-value is allowed by the law. Elections Canada is fully aware of this practice," she wrote.

Bob Gallagher, a spokesman with the United Steelworkers told The Huffington Post Canada Sunday that Steelworkers are “very aware” that they cannot donate to the party or candidates.

“As far as I know, all the items that were paid were for things like advertising and sponsorship that were paid at a fair market value,” he said, later adding in an email that this “is allowed by law.”

CUPE also notes in its 2011 election financing fact-sheet, that despite being banned from contributing money, goods or services to any federal political party, “CUPE National, a Division or a CUPE Local can sponsor events and conventions so long as the fair market value is paid for the services that are associated with the sponsorship.”

Hamilton concluded his letter by requesting that the rules surrounding contributions by unions and incorporated entities, and on what constitutes a material provision, be clarified.

Elections Canada could not be reached for a comment.

If the rules governing sponsorship change or their interpretation changes, NDP officials may soon want to know. They are currently looking at budgets and planning for next year's leadership convention. The party’s senior executives are scheduled to meet Wednesday to recommend the rules that will govern the contest. They will be approve by the NDP's larger federal council on Friday.

SLIDESHOW: Images of convention signs included with Hamilton's letter.